1. Field of the Invention
The present invention relates to a femto Base Station (BS) for reducing Inter-Cell Interference (ICI) and a method for transmitting a signal using the same.
2. Discussion of the Related Art
Traditional cellular systems have been developed along two major axes, 3rd Generation Partnership Project (3GPP) based on Global System for Mobile communications (GSM) and 3GPP2 based on Code Division Multiple Access (CDMA). Recently, the Worldwide Interoperability for Microwave Access (WiMAX) camp has joined the development. The most critical factor for cellular wireless communications is high-speed throughput. The recent soaring increase in wireless communication throughput is attributed to the increase of Signal-to-Interference and Noise Ratio (SINR) as a result of scaling down a cell size.
One of up-to-date technologies that increase SINR through cell size reduction is femtocell. The term ‘femtocell’ generically refers to an ultra small BS covering tens of meters which a user can install at home or in an office, the resulting ultra small cell, and associated technologies. An ultra small BS for a femtocell may be referred to as a femto BS, a femto Access Point (AP), a femto Advanced BS (ABS), a femtocell BS, or a femtocell. Herein, such an ultra small BS is referred to as a femto BS or a femtocell BS. In contrast, a conventional BS having a wide coverage may be referred to as a macro BS or a macrocell BS.
A femto BS may be installed in a shadowing area that a macro BS cannot cover. The femto BS is a small-size version of a macro BS, capable of carrying out most of the functions of the macro BS. The femto BS has a network configuration that operates independently. It is expected that far more femto BSs than relay BSs will be installed downtown or indoors. Accordingly, a neighbor BS list that a BS transmits to a User Equipment (UE) does not contain a femto BS list because the femto BS list has too large an amount of information.
FIG. 1 illustrates the configuration of a wireless communication system to which a femtocell BS is added.
Referring to FIG. 1, the wireless communication system includes a femtocell BS 110, a macrocell BS 120, a Femto Network Gateway (FNG) 130, an Access Service Network (ASN) 140, and a Connectivity Service Network (CSN) 150. The macrocell BS 120 is a general BS in a conventional wireless communication system.
The femtocell BS 110 is connected directly to a Transmission Control Protocol/Internet Protocol (TCP/IP) network and operates independently, like the macrocell BS 120. The femtocell BS 110 has a coverage of 0.1 to 30 m and accommodates 10 to 20 MSs. The femtocell BS 110 may operate at the same frequency as or a different frequency from the macrocell BS 120.
The femtocell BS 110 may be connected to the macrocell BS 120 via an R1 interface and thus may receive a downlink channel from the macrocell BS 120 or transmit a control signal to the macrocell BS 120 via the R1 interface.
The femtocell BS 110 may cover an indoor area or shadowing area that the macrocell BS 120 cannot afford to cover and may support high-speed data transmission. The femtocell BS 110 may be overlaid within the macrocell of the macrocell BS 120 or installed outside the coverage of the macrocell BS 120 in a non-overlay fashion.
There are two types of femto BSs, Closed Subscriber Group (CSG) femto BSs and Open Subscriber Group (OSG) femto BSs. A CSG femto BS groups MSs that can access the CSG femto BS and assigns a CSG Identification (ID) to the MSs. Thus only the MSs having the CSG ID can access the CSG femto BS. On the other hand, an OSG femto BS is accessible to all MSs. A hybrid femto BS type may further be defined by combining the features of a CSG femto BS with the features of an OSG femto BS.
The FNG 130, a gateway that controls the femto BS 110, is connected to the ASN 140 and the CSN 150 via an Rx interface and an Ry interface, respectively. The femto BS 110 may receive a service from the CSN 150 through the FNG 130 and a Mobile Station (MS) connected to the femto BS 110 may receive services such as authentication, IP Multimedia Subsystem (IMS) service, etc. from the FNG 130 or the CSN 150. The CSN 150 provides connectivity of application services such as Internet, Voice over Internet Protocol (VoIP), etc. and authentication and billing functions to MSs. The ASN 140 controls the macrocell BS 120 and manages connection between the macrocell BS 120 and the CSN 150.
It is anticipated that interference will get more severe than in a conventional environment, in a femtocell environment. One reason for the increased interference is that interference factors increase in number due to interference from femtocells in addition to interference between macrocells. Another reason is that a large number of femtocell BSs will be installed on a user basis, compared to macrocell BSs whose installation is planned by service providers. A third reason is that it will be more difficult to directly control femtocell BSs than macrocell BSs.
Conventional studies on control of interference from femtocells are classified into techniques for using a different frequency band from a macrocell or a neighbor femtocell for a femtocell and techniques for adaptively controlling the power of a femtocell BS or a user.
Another technique for mitigating interference caused by femtocells is to discontinue signal transmission of a femtocell BS under circumstances. For example, a femtocell BS installed at home or in an office may have no users to serve in a specific time zone of the daytime or the night. If the femtocell BS continues to transmit a pilot signal nonetheless, another BS and an MS that measure interference using a pilot signal use only limited resources even though they are capable of using higher power or a wider bandwidth. This is called a pilot pollution problem.
To avert the pilot pollution problem, a method for optimizing a cell size by controlling transmission power has been proposed. However, this method is an approach to increasing spatial reuse, rather than a solution to pilot pollution. The Long Term Evolution (LTE) or Mobile WiMAX standardization organization interested in the femtocell technology clarifies an inactive mode (e.g. idle mode) for a femtocell BS in a standard, without specifying an algorithm for solving the pilot pollution problem.
Hereinbelow, the active/inactive mode of a femtocell BS designed to solve the pilot pollution problem and femto BS/UE deadlock resulting from the active/inactive mode of the femtocell BS will be described.
An additional description will be given of embodiments of the present invention to solve the femto BS/UE deadlock problem.